1. Field of the Invention
This invention relates generally to a system and method for estimating the concentration of hydrogen and/or nitrogen in a fuel cell system at system shut-down and start-up and, more particularly, to a system and method for estimating the concentration of hydrogen and/or nitrogen in a fuel cell system at system shut-down and start-up that includes defining the fuel cell system into an anode flow-field and plumbing volume, a cathode flow-field volume and a cathode header and plumbing volume and calculating the fluxes of hydrogen, nitrogen, oxygen and/or water into and out of the volumes.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
At system start-up, assuming enough time has gone by since the previous shut-down, most of the hydrogen remaining in the stack at the last shut-down has diffused out of the stack and both the cathode and anode flow channels are generally filled with air. When hydrogen is introduced into the anode flow channels at system start-up, the hydrogen pushes the air out of the anode flow channels creating a hydrogen/air front that travels through the anode flow channels. As described in the literature, the presence of the hydrogen/air front on the anode side combined with air on the cathode side causes a series of electrochemical reactions to occur that result in the consumption of the carbon support on the cathode side of the MEA, thereby reducing the life of the MEAs in the fuel cell stack. For example, it has been shown that without addressing the degradation effects of the hydrogen/air front at system start-up, it is possible for about 100 shut-down and start-up cycles to destroy the fuel cell stack in this manner.
One known technique for significantly reducing the air/hydrogen front at system start-up, and thus, reducing catalytic corrosion is to reduce the frequency of start-ups in which the anode and cathode are filled with air. One strategy to achieve this is to leave the anode and cathode in a nitrogen/hydrogen environment. However, the hydrogen will eventually either diffuse out of the anode, or be consumed by oxygen slowly returning to the stack. Thus, in order to extend the ability to reduce catalytic corrosion, hydrogen can be periodically injected into the stack while the system is shut-down. Because mostly nitrogen is remaining in the cathode side at system shut-down, as the result of the oxygen being consumed by the fuel cell reaction, nitrogen and hydrogen are the main elements that are equalized in the cathode and anode sides of the fuel cell stack after system shut-down. This does not allow air including oxygen to form the air/hydrogen front.
When the fuel cell system is shut down, the gas permeation continues through the membrane until the gas component partial pressures have equalized on both sides of the membrane. The diffusivity of hydrogen through the membrane from the anode to the cathode is approximately three times the rate of nitrogen from the cathode to the anode. Higher hydrogen diffusivity rates equate to a rapid equalization of hydrogen partial pressures compared to a relatively slow equalization of nitrogen partial pressure. The difference in gas diffusivities causes the anode sub-system absolute pressure to drop until the cathode hydrogen partial pressure reaches the anode hydrogen partial pressure. Typically, the anode side of the fuel cell stack is operated at a high hydrogen concentration, such as greater than 60%, and large volumes of hydrogen-rich gas exist in the anode headers and anode plumbing outside of the anode of the stack. As the anode absolute pressure drops, more hydrogen is drawn out of the anode sub-system into the anode flow field of the stack.
The net result of the hydrogen partial pressure equalization after system shut-down is an increase in the concentration of hydrogen in the cathode side of the fuel cell stack with time, at least for some period of time after shut-down. At system start-up, the compressor is started, but the concentration of hydrogen exiting the fuel cell stack from the cathode must be limited so as to not violate emission requirements. Thus, as the cathode of the fuel cell is filled with fresh air, the hydrogen rich gas leaving the cathode side of the stack must be diluted. To meet start-time and noise requirements, there is a need to optimize the fill time of the stack cathode. Because the cathode flow is limited by the power available to the compressor, the fill method must be robust to changes in total compressor flow rate.
It is desirable to predict or estimate the amount of hydrogen in the anode and cathode of a fuel cell system during system start-up to allow the start-up strategy to meet emissions requirements while maximizing reliability and minimizing start time. It is generally desirable that the hydrogen concentration estimator be robust to shut-down and off time related functions and account for membrane permeation of gases as well as air intrusion from external sources. At the same time, the estimation algorithm must be simple enough to be provided in an automotive controller with the calculation sufficiently minimal so as to be completed without delaying the start-up.
Determining the hydrogen concentration in the anode and cathode of the fuel cell stack at start-up will allow the fastest possible start time because the system control does not need to provide excess dilution air when unnecessary. Further, knowing the hydrogen concentration provides a more reliable start because the amount of hydrogen in the anode that needs to be replenished will be known. This is especially relevant for start-ups from a stand-by state, or from the middle of a shut-down, where hydrogen concentrations can be relatively high.
Further, knowing the hydrogen concentration improves durability because when there is an unknown hydrogen concentration in the stack, typical start-up strategies assume the worst case percentage of hydrogen for injection purposes and 100% hydrogen for dilution purposes. In those situations, the initial anode flush with hydrogen could be slower than if the stack is known to be filled with air. The rate of corrosion is proportional to the initial hydrogen flow rate. Therefore, without accurately knowing the hydrogen concentration, each of these events will be more damaging than necessary.
Also, knowing the hydrogen concentration provides improved efficiency because a more accurate determination of hydrogen concentration in the anode and cathode prior to start-up will lead to more effective start-up decisions and potential reduction in hydrogen uses. For example, dilution air could be lowered if it is known that the stack is starting with no hydrogen in it. Further, knowing the hydrogen concentration provides more robust start-ups. In the event of a premature shut-down or a shut-down with a failed sensor, the algorithm can use physical limits to provide an upper and lower bound on the hydrogen in the cathode and anode.